Heretofore, it was known that, in heat treatment, steel could be hardened to the desired degree within certain limits by controlling the dispersion of iron carbide in ferrite or by forming a metastable solid solution of carbon in body-centered tetragonal iron. It has also been known that the finer the dispersion, the harder the steel.
Notwithstanding the foregoing, for many years the properties of other alloys and metals could not be altered except by cold working and annealing to produce recrystallization. Then, in 1911 there was published the results of tests on a certain aluminum allow revealing that the strength of the particular aluminum allow involved increased with aging at room temperature after a quenching treatment. Since that discovery, the change in properties on aging after suitable treatment was observed in many alloys and was put to practical use. The phenomenon of the increase of hardness and strength as a function of time is called "age hardening".
A probable explanation of the phenomenon has given rise to the use of the term "precipitation hardening". This term appears to be adequate even though the precise mechanism by which this hardening takes place is not tool well understood.
The treatment of alloys for precipitation hardening requires very careful control, because in most cases the temperature at which the homogeneous solid solution is attainable lies within a very narrow range. Burning, embrittlement, or actual fusion may result from heating above this temperature. On the other hand, too low a temperature will permit the complete separation of a portion of the second phase in the grain boundaries, thus preventing the attainment of maximum hardening. Complete homogenization at the proper temperature requires proper timing. This process is known as "solution heat treating". The type of alloy being treated, the section of the part, and the temperature employed will determine the time element involved. The time may vary from ten to fifteen minutes up to a matter of several days. With some, if not most, alloys it is necessary to transfer the alloy parts from the heating furnace to the quenching bath in a very short time. Gross precipitation of the phase which is to bring about hardening may occur if there is delay in this transfer. In some alloys, such as nickel-silicon and iron-copper, precipitation does not occur very rapidly. Therefore, air cooling may be sufficiently rapid so that maximum hardening can be attained by artificial aging.
The precipitation treatment which is a combination of aging temperature and aging time, is rather critical for some alloys. As a general rule, a decrease of the aging temperature requires a considerable increase of the time of aging. Lower aging temperatures bring about appreciable increases in strength without materially decreasing the ductility in some alloys. This is evident in some aluminum-copper alloys.
As indicated hereinabove, the solution heat treating for aluminum alloys has been known for many years. Heat treatable aluminum alloys are generally heated in a range of from 750.degree. F to as high as approximately 1000.degree. F. This is followed by an aqueous solution quench, or forced air cooling of the aluminum product. This in turn is followed by a room temperature aging process, or an elevated temperature artificial aging process to obtain full physical properties from the alloys.
Gas-heated or electric-heated forced convection ovens have been used for the solution heating as well as artificial aging. The conventional aqueous quenching tanks are usually closely adjacent to the solution heat treating oven so that a minimum time is required to transfer the aluminum alloy from the solution heat treating to the quench.
Molten salt bath furnaces have been used for the solution heat treating requirements. Usually, a binary mixture of sodium nitrate and potassium nitrate at temperatures between 750.degree. F and 1000.degree. F are used. The conventional aqueous quench tanks are adjacent to the salt bath furnace to minimize transfer time from the solution heat treat furnace. Conventionally, forced air convection ovens are used for the aging treatment.
In recent years, the aluminum industry has developed aluminum alloys which produce desirable physical properties but do not require rapid cooling. Examples of such newer aluminum alloys are Alcoa Aluminum Alloy 7005 and X-7046 both of which are heat-treatable aluminum-zinc-magnesium alloys. Alloy 7005 is disclosed in "Alcoa Green Letter: Alcoa Aluminum Alloy 7005", GL 198 (Rev. 1-75) and by U.S. Pat. No. 3,304,209. Alloy X-7046 is disclosed by an Alcoa publication entitled "Bumpology", E28-13534. The usual heat treatment practice for alloy X-7046 extrusions and sheet comprises: solution heat treat for fifteen minutes at 750.degree. F plus or minus 50.degree. F in a continuous furnace; fan quench at a minimum rate to exceed 4.degree. F per second; natural age at room temperature for eight hours minimum; and artificially age for three hours at 200.degree. F plus or minus 10.degree. F, followed by eight hours at 275.degree. F plus or minus 10.degree. F.
The above-mentioned Alloy 7005 may be thermally treated with the process disclosed by U.S. Pat. No. 3,171,760 which is believed to involve a cooling rate from 750.degree. F in a forced air quench system of approximately 10.degree. F per second, followed by a three day aging at room temperature, which in turn is followed by an artificial aging cycle.
The prior art is further exemplified by U.S. Pat. Nos. 3,304,209; 3,414,406 and 3,868,279.
The advantage of the air quench thermal treatment process to the aforesaid newer alloys is that it minimizes the distortion of the fabricated assemblies by virtue of the fact that the treatment eliminates the need for aqueous quenching. However, one of the disadvantages is the long-natural and artificial aging time requirements. In addition, with respect to the alloy X-7046, full physical properties on section thicknesses is required for many components, such as automotive bumper components, and are not necessarily being met due to the inability of cooling the alloy rapidly enough with forced room temperature air. These properties can be improved by aqueous quenching, but this causes distortion which is not acceptable. Such distortion could be minimized by re-striking the components on appropriate dies, but this involves an additional operation which adds to the total cost of the process.